The present invention relates to a silicone rubber composition superior in heat resistance, and, in particular, relates to a silicone rubber composition superior in storage stability during the silicone rubber base compound stage and, upon curing, is capable of producing silicone rubber that has a low degree of thermal degradation and is superior in heat resistance.
The technique of admixing cerium oxide powder into silicone rubber as a heat stabilizer is known in the art (for example, see Japanese Kokai (Unexamined) Patent Publication No. Sho 52(1977)-14654, Japanese Kokai (Unexamined) Patent Publication No. Sho 60(1985)-173050, and Japanese Kokai (Unexamined) Patent Publication No. Hei 07(1995)-11010). Incidentally, it is commonly believed that it is difficult to compound cerium oxide powder with silicone rubber compositions in a uniform manner. The term "silicone rubber compositions" here means silicone rubber compositions prepared in such a manner that a silicone rubber base compound is first made by mixing diorganopolysiloxane polymer, reinforcing fillers, and, if necessary, various additives used for improving the physical properties of the silicone rubber composition, and the silicone rubber base compound is then stored for future use, allowing for curing by admixing a curing agent at the time of molding articles from the silicone rubber. Compounding the cerium oxide powder with the silicone rubber composition in a uniform manner during the stage of silicone rubber base compound preparation presents certain difficulties, however. For example, the problem that arises when a cerium oxide powder with a large specific surface area and a small particle size is added and kneaded during the stage of silicone rubber base compound preparation is that heat is released and the viscosity of the silicone rubber base compound itself increases, resulting in gelling in severe cases. Furthermore, the problem is that silicone rubber base compounds with cerium oxide powder added thereto have inferior storage properties, and when stored over an extended period of time, their viscosity increases while their rollability characteristics decrease, with gelling occurring in severe cases.
As a result of in-depth investigations aimed at eliminating the above described problems the inventors arrived at the present invention by discovering that a silicone rubber base compound obtained by compounding a special cerium oxide powder is superior in storage stability and that, moreover, silicone rubber obtained by curing a silicone rubber composition produced by compounding a curing agent with the silicone rubber base compound having such a special cerium oxide powder compounded therewith exhibits a low degree of thermal degradation under high temperature conditions exceeding 200.degree. C.
Namely, it is an object of the present invention to provide a silicone rubber composition that is capable of producing silicone rubber superior in storage stability during the silicone rubber base compound stage, and, upon curing, exhibiting a low degree of thermal degradation under high temperature conditions and is superior in heat resistance.